Using new instrumentation and procedures, the b-cytochromes of beef heart mitochondria have been characterized in terms of spectra and thermodynamic properties. In place of two 1-electron species, we find four species, two of which pass more than a single electron. Three of the four species change midpoint potentials 60 mV/pH unit. None of the cytochromes respond to ATP or antimycin. An apparent red shift in the spectrum of cytochrome b-k caused by antimycin seems to be due to an alteration in the accessibility of one of the newly described cytochromes to reducing agents. A fundamental thermodynamic flaw in Q-cycle mechanisms of electron transport has been uncovered and analyzed.